"We haven't heard anything about him since he went there," said the activist, a member of the Seoul-based group Pax Koreana, which promotes human rights in the North. "But it's North Korea. We presume he has been arrested there."
Analysts said North Korea would most likely expel the Christian missionary instead of holding him as a negotiating card because that would serve his purpose of highlighting the regime's human rights abuses.
The communist country's state-run media did not mention Park's case Sunday. The U.S. State Department and the U.S. Embassy in Beijing said they were aware of the incident but had no details.
Park, who is of Korean descent, was carrying a letter urging leader Kim Jong Il to step down and free all political prisoners, the activist said.
"I am an American citizen. I brought God's love. God loves you and God bless you," Park reportedly said in fluent Korean as he crossed over near the northeastern city of Hoeryong, according to the Pax Koreana activist.
Two other activists filmed Park cross into the North, the activist told The Associated Press, speaking on condition of anonymity because of the sensitivity of the situation.
Park's crossing comes just months after North Korea freed two U.S. journalists who had been arrested in March and sentenced to 12 years of hard labor for trespassing and "hostile acts." Pyongyang dexterously used their detention as a negotiating card with Washington amid a standoff over its nuclear programs.
North Korea waited four days before announcing on March 21 that they had been detained.
Former President Bill Clinton visited Pyongyang in August to bring them home. That trip, which included a meeting between Clinton and Kim Jong Il, led to the first high-level talks between the two countries earlier this month since President Barack Obama took office.
North Korea watchers in Seoul, however, said Park's case was unlikely to develop in the same way.
"I think it will end up an isolated episode," said Kim Yong-hyun, an expert on North Korea at Seoul's Dongguk University. "North Korea knows that it would be serving the purpose of the activist and highlight its human rights problems if it keeps holding him like it did the journalists."
Analyst Hong Hyun-ik at the private Sejong Institute think tank near Seoul also said the North was not expected to pay much attention to Park's action, which he described as "Don Quixote-like." Hong said Pyongyang was likely to expel him, a view echoed by Yoo Ho-yeol at Seoul's Korea University.
North Korea's criminal code punishes illegal entry with up to three years in prison, but the general view of analysts is that the North must see some political gain in keeping high-profile foreigners prisoner.
In this case, holding Park might bring attention to his cause.
Park, from Tucson, Arizona, carried a letter to Kim Jong Il calling for major changes to his totalitarian regime, according to the Pax Koreana activist.
"Please open your borders so that we may bring food, provisions, medicine, necessities, and assistance to those who are struggling to survive," said the letter, according to a copy posted on the conservative group's Web site. "Please close down all concentration camps and release all political prisoners today."
North Korea holds some 154,000 political prisoners in six large camps across the country, according to South Korean government estimates. Pyongyang has long been regarded as having one of the world's worst human rights records, but it denies the existence of prison camps.
In a second letter, Park calls for Kim to immediately step down, noting starvation, torture and deaths in North Korean political prison camps, according to the activist.
Other activists said Park had become known over the last year in Seoul human rights circles for his religious fervor and passion for helping North Koreans. And not all analysts see a simple end to the drama.
Koh Yu-hwan, another North Korea expert at Dongguk University, said Park's illegal entry would take time to resolve because the North will see his demanding Kim step down as "a kind of hostile act."